Corrosion-resistant alloys



US. Cl. 75ll71 7 Claims ABSTRACT OF THE DISCLOSURE A castable, weldable,machinable, air-meltable alloy which is highly resistant to corrosionunder widely divergent conditions in various corrosive media includingoxidizing media and reducing or non-oxidizing media. The alloy iscomposed essentially of about 14.7 to 18.0 percent by weight chromium,about 3.5 to 9.0 percent by weight tantalum, about 15 to 18 percent byweight molybdenum, up to about 0.15 percent by weight carbon, up toabout 1.50 percent by weight silicon, up to about 1.50 percent by weightmanganese, up to about 5.25 percent by weight tungsten, up to about 7.0percent by weight iron, up to about 0.35 percent by weight vanadium, upto about 10.5 percent by weight cobalt, up to about 4 percent by weightcopper, up to about 4 percent by weight columbium, and the balancenickel.

Background of the invention This invention lies in the field of alloysand more particularly in the field of corrosion-resistant alloys whichdisplay a high degree of resistance to corrosion in various corrosivemedia and which are weldable, machinable and may be produced in wroughtor cast form.

In recent years, the rapidly expanding chemical and petrochemicalindustries have created a demand for metals or alloys suitable for usein environments destructive to the standard types of stainless steels.Corrosive media encountered in industrial equipment and processes may,in general, be of the oxidizing type or non-oxidizing (or reducing)type. By an oxidizing environment is meant one more oxidizing than thehydrogen ion. By a reducing environment is meant one which does notcontain any substance more oxidizing than the hydrogen ion.

Among oxidizing agents which have caused corrosion problems in the pastmay be mentioned nitric acid, ferric chloride, aqua regia, cupricchloride, hydrogen peroxide and sodium hypochlorite. Non-oxidizing orreducing media include those which do not afford the chemical passivitywell known in the industry to be associated with oxidizing media andinclude such media as hydrochloric acid, concentrated sulfuric acid,phosphoric acid, oxalic acid, acetic acid, and aluminum chloride. Ingeneral, the aggressive agents of this group readily atack and destroymetallic alloys which are virtually immune to many severely-oxidizingmedia. Conversely, many oxidizing agents readily attack those metallicalloys which are most resistant to aggressive non-oxidizing media. Thepresence of halide ions, particularly chloride ions, further aggravatesattack on most alloys in many applications. Thus, for example, standardstainless steels resist most concentrations of nitric acid very well butare completely dissolved in time by aqua regia (one part nitric acid andthree parts hydrochloric acid).

Metals or alloys which are resistant to corrosion under both oxidizingand reducing conditions are highly desirable in the industrial equipmentfield inasmuch as they afiord the designer and equipment builder agreater degree of latitude and render the equipment more flexible inuse. Further, since process conditions may unexpectedly change theenvironment involved from reducing to oxidiz nited States Patent3,473,922 Patented Oct. 21, 1969 "ice the resistance of titanium tocorrosion is considerably en-;

hanced, particularly in a reducing environment in which unalloyedtitanium may fail rapidly. However, such titanium alloys cannot befurnished in cast form and must be vacuum melted and prepared from veryhigh purity materials. Also, such alloys must be welded in an inert gasatmosphere. Similarly, an alloy consisting of 50 percent by weightvanadium and 50 percent by weight columbium has been found to haveappreciably greater ability to withstand oxidizing or reducing acids inthe presence or absence of chlorides over pure vanadium, which resistsreducing agents with or without chlorides but does not resist oxidation.Here again, however, such an alloy is available only in wrought form,not in cast form, is quite expensive and must be vacuum melted from highpurity stock. It is also welded only with extreme difiiculty. There hasremained, therefore, an unfulfilled need for alloys which are resistantto corrosion under both oxidizing and reducing conditions, with orwithout the presence of halide ions and which may be prepared from lowercost materials and supplied in either cast or wrought form.

Summary of the invention Among the several objects of the invention maythus be noted the provision of novel alloys which are highly resistantto corrosion under widely varying conditions, including oxidizing andnon-oxidizing environments; the provision of such alloys which may beprovided in either cast or wrought form; the provision of alloys of thistype which are weldable, machinable and air-meltable; the provision ofsuch alloys which need not be subjected to high temperature solutionheat treatment for service in most corrosive media; and the provision ofalloys of thi character which may be readily prepared by conventionalmethods from lower cost materials. Other objects will be in partapparent and in part pointed out hereinafter.

The present invention is therefore directed to a cor rosion-resistant,weldable, machinable alloy characterized by its ability to produceusable cast products, the alloy being composed essentially of from about14.7 to 18.0 percent by weight chromium, about 3.5 to 9.0 percent byweight tantalum, about 15 to 18 percent by weight molybdenun, up toabout 0.15 percent by weight carbon, up to about 1.50 percent by weightsilicon, up to about 1.50 percent by weight manganese, up to about 5.25percent by weight tungsten, up to about 7.0 percent by weight iron, upto about 0.35 percent by weight vanadium,

Description of the preferred embodiments In accordance with the presentinvention, I have found that alloys which are highly corrosion resistantin various environments may be formed from a solid solution composedessentially of 14.7 to 18 percent chromium, 3.5 to 9.0 percent tantalum,15 to 18 percent molybdenum, and the balance nickel. The alloys of theinvention may also contain up to about 0.15 percent carbon, up to about1.50 percent silicon, up to about 1.50 percent manganese, up to about5.25 percent tungsten, up to about 7.0 percent iron, up to about 0.35percent vana dium, up to about 10.5 percent cobalt, up to about 4percent copper and up to about 4 percent columbium, The

alloys may be prepared using conventional electric furnace, either arcor induction, procedures. The components of my alloys are employed insubstantially pure form in preparing the alloys, but the usualimpurities present in commercially available metals are acceptable anddo not adversely affect the properties of the alloys produced. When ironis included in the alloys of the invention, then the tantalum can beadded in the form of ferro-tantalum. Similarly, molybdenum may be addedin the form of ferro-molybdenum provided the iron content of the alloyis not permitted to exceed about 7 percent. Silicon and manganese may beincluded as deoxidizing agents to avoid loss of tantalum from the meltbut may be omitted when vacuum melting is employed. In order to attain ahigh degree of corrosion resistance, the following composition range hasbeen found satisfactory and preferable:

Percentage by weight Chromium 14.7-18.0 Tantalum 3.5-9.0 Molybdenum15-18 Carbon 0.02-0.15 Silicon 0.3-1.5 Manganese 0.3-1.5 Tungsten -5.25

Iron 0-7.0 Vanadium 0-0.35 Cobalt 0-10.5 Copper 0-4 Columbium 0-4 NickelRemainder When iron is included, the alloys preferably contain about3.3-3 :84 percent iron. When vanadium is included, about 0.20-0.22percent may be present. While it is preferable to formulate my alloyswithout cobalt, the alloys may include up to 10.5 percent cobalt,preferably l.1-10.5 percent. In general, the copper content should beheld below 4 percent since this component may be detrimental in somemedia. Similarly, the amount of cobalt should be held below about 10.5percent, the amount of columbium below about 4 percent and the amount oftungsten below about 5.25 percent. While these elements may be toleratedup to the stated amounts, preferably they are omitted where feasible inthe practice of the invention.

It is to be noted that the tantalum content of my alloys is greater thanthat required to merely combine chemically with and stabilize the carbonwhich has been the function of tantalum (or columbium) in certain priorart alloys. Thus, the tantalum content of the alloys of the inventionexceeds the amount necessary for car-bide stabilization so that theexcess amount remains in solid solution in the alloy and functionessentially as a noble metal.

In addition to exhibiting remarkable resistance to corrosion in variouscorrosive media, including both oxidizing and reducing types of media,the alloys of the invention provide a number of significant advantagesnot pre- 'viously attainable with alloys possesing a high degree ofcorrosion resistance. Thus, my alloys may be produced in both wroughtand cast form and may be heat treated in the conventional manner. Ingeneral, the alloys of the invention in a solution heat treatedcondition exhibit a higher degree of corrosion resistance in some mediathan do the alloys in an as-cast condition. In both forms however, thealloys possess improved corrosion resistance as demonstrated by the testwork described hereinafter. Moreover, the alloys are weldable andmachinable using conventional techniques. Also, the alloys areair-meltable and therefore need not be vacuum melted, and likewise neednot be prepared from high purity materials. Finally, the alloys of theinvention display a high degree of hardness and when tested, showBrinell hardness number values of 250 to 300. Accordingly, the alloysare usable in many 'H SO -'non-oxidizing medium with no halide ions 4applications and environments in which currently availablecorrosion-resistant alloys are incapable of use or have seriousdrawbacks.

The following examples further illustrate the invention.

The alloys set forth inTable I below were prepared utilizingconventional electric furnace-procedures.

TABLE I Percentage by Weight Alloying Alloy No. 1 Alloy N o. 2 Alloy No.3 Alloy No. 4 Element Chromium 16. 08 17. 35 17. 19 L4. 9

Tantalum 5. 76 4. 44 8. 93 ll. 79

Molybdenum... 17.45 16. 56 16. 40 t8. 0

3. 3. 84 3. 43 L ll. 30

Each of the above alloys was air melted and poured in batches of 500pounds to 1,000 pounds using a high frequency induction furnace. Alloysfrom each of the heats were cast into well-risered cylinders of 5%."length and 2% diameter. Samples of each alloy were their solution heattreated by being brought up to 2225 F. for one hour, water quenched to1000 F. and then air cooled.

The as-cast and heat treated cylinders were then cut into discs ofapproximately 1.750 inches diameter and 0.25 inch thickness, ground to a600 grit and lightly polished with a diamond dust lapping paste to afine micro-finish. Each sample disc was provided with a inch holethrough the middle in order to facilitate sample suspension in the testsolutions described below.

The chemical testing apparatus employed consisted of a water bathprovided with a, heater, the temperature of .the Water being controlledto within plus or minus 3 F.- by abuilt-in thermostat. The entire unitwas enclosed in a polyethylene hood which was provided witha smallexhaust fan to remove corrosive gases.

The, test solutions were aerated by having air bubbled therethrough fromindividual glass .tube lines with each line being provided with acontrol stop cock. The flow of air was suflicient to keep the aeratedsolutions in constant agitation. r

The test disc. specimens were suspended by platinum wires hanging fromglass hooks and immersed in watchglass covered beakers containing thevarious corrosive media employed in the tests. Liqiud media condensingon the respective Watch glasses dripped back into the beakers therebypreventing evaporation and changes in the concentration of the testsolutions.

The following test solutions were employed in the test These corrosivemediawere selected to provide examples of each of the following widelydivergent conditions:

- v Percent CuCl oXidizing medium with halide ions 30 HNO oxidizingmedium with no halide ions 70 HCl-non-0xidizing medium with halide ions37 25 The test solutions were maintained at a temperature of F. incarrying out the tests, and the test period was 24 hours. v

Samples of, commercially available alloys marketed under the tradedesignations Hastelloy C3 Hastelloy B," Durimet 20" and *Monel wereprepared as described above and subjected to corrosion tests in certainof the corrosive media. The Monel alloy used consisted of 65 percentnickel, 30 percent copper, 2 percent iron, 1.5 percent silicon, 0.8percent manganese and 0.2 percent carbon. The Durirnet alloy usedconsisted of 41 percent iron, 30 percent nickel, 20 percent chromium, 4percent copper, 3 percent molybdenum, 0.8 percent silicon, 0.7 percentmanganese and 0.05 percent carbon. The Hastelloy B alloy used consistedof 61 percent nickel, 28 percent molybdenum, 4 percent iron, 0.3 percentvanadium, 0.5 percent silicon, 0.5 percent manganese and 0.04 percentcarbon. The Hastelloy C alloy used consisted of 52.6 percent nickel, 18percent molybdenum, 16.2 percent chromium, 5.3 percent tungsten, 3.9percent iron, 0.6 percent cobalt, 0.5 percent silicon, 0.3 percentmanganese and 0.08 percent carbon. These alloys are used commercially inequipment for handling corrosive materials and exemplify alloys whichresist corrosion in certain media but fail catastrophically in othermedia.

The results of the tests are set forth in Table 11 below.

3. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 14.9 to 17.35 percent by weight chromium,about 3.79 to 8.93 percent by weight tantalum, about 16.4 to 18.0percent by weight molybdenum, about 0.06 to 0.13 percent by weightcarbon, about 0.46 to 0.82 percent by weight silicon, about 0.3 to 0.44percent by weight manganese, about 3.3 to 3.84 percent by weight iron,about 0.20 to 0.22 percent by weight vanadium, about 1.1 to 10.5 percentby Weight cobalt, and the balance nickel.

4. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 16.08 percent by weight chromium, about 5.76percent by weight tantalum, about 17.45 percent by weight molybdenum,about 0.13 percent by weight carbon, about 0.52 percent by weightsilicon, about 0.33 percent by weight TABLE II Solution lI-eat-Clreated3 Condition, Inches Per Year As-Cast Condition, Inches PerYear Corrosion Ol'IOS101'1 Alloy Unaerated Aerated Unaerated AeratedUnaerated Aerated Unaerated Aerated 30% CuCl 70% END; 37% HCl H SO01.1015 70% NBC; 37% B01 25% H 804 Alloy No. 1 0. 0008 0. 0154 0. 01090. 0003 0. 020 0. 0170 0. 0369 0. 0081 Alloy No. 2. Nil 0. 0213 0. 04450. 0001 0. 0587 0. 0166 0. 0744 0. 0002 Alloy N0. 3. Nil 0. 0247 0. 02220. 0001 0. 7803 0. 0206 0. 0520 0. 0003 Alloy No 4 0. 0465 0. 0170 0.0204 0. 0004 1. 062 0. 0194 0. 0245 0. 0048 Hastelloy C 0. 8484 0. 0440. 0369 0. 006 1. 7588 0. 0451 0. 0283 0. 0235 Hastelloy B Bad 0. 002Durimet 20 Bad 0. 005 Bad Monel Bad Bad Bad In the above tests, thelegend Bad designates that the specimen failed catastrophically, eitherbeing completely dissolved or almost completely dissolved, so that nomeasurement could be made.

From the foregoing, it will be seen that the present invention providesalloys of improved corrosion resistance under varying corrosiveconditions, such alloys being weldable, machina'ble, air-meltable andreadily castable. While displaying these desirable properties, thealloys of the invention are nevertheless formed from readily avail-'able and comparatively inexpensive non-precious metals of commercialpurity and may be produced economically utilizing conventionaltechniques.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

What is claimed is:

1. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 14.7 to 18.0 percent by weight chromium, about3.5 to 9.0 percent by weight tantalum, about 15 to 18 percent by weightmolybdenum, up to about 0.15 percent by weight carbon, up to about 1.50percent by weight silicon, up to about 1.50 percent by weight manganese,up to about 5.25 percent by weight tungsten, up to about 7.0 percent byWeight iron, up to about 0.35 percent by weight vanadium, up to about10.5 percent by weight cobalt, up to about 4 percent by weight copper,up to about 4 percent by weight colurnbium, and the balance nickel.

2. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 14.7 to 18.0 percent chromium, about 3.5 to9.0 percent by weight tantalum, about 15 to 18 percent by Weightmolybdenum, about 0.02 to 0.15 percent by weight carbon, about 0.3 to1.5 percent by weight silicon, about 0.3 to 1.5 percent by weightmanganese, up to about 5.25 percent by weight tungsten, up to about 7.0percent by weight iron, up to about 0.35 percent by weight vanadium, upto about 10.5 percent by weight cobalt, up to about 4 percent by weightcopper, up to about 4 percent by weight columbium, and the balancenickel.

manganese, about 3.8 percent by Weight iron, about 1.1 percent by weightcobalt, and the balance nickel.

5. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 17.35 percent by Weight chromium, 4.44 percentby weight tantalum, about 16.56 percent by weight molybdenum, about 0.06percent by weight carbon, about 0.82 percent by weight silicon, about0.44 percent by weight manganese, about 3.84 percent by weight iron,about 0.20 percent by weight vanadium, about 8.36 percent by weightcobalt, and the balance nickel.

6. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 17.19 percent chromium, about 8.93 percent byweight tantalum, about 16.40 percent by Weight molybdenum, about 0.08percent by weight carbon, about 0.46 percent by weight silicon, about0.36 percent by weight manganese, about 3.43 percent by weight iron,about 0.22 percent by weight vanadium, and the balance nickel.

7. A corrosion-resistant, weldable, machinable alloy characterized byits ability to produce usable cast products, said alloy being composedessentially of from about 14.9 percent chromium, about 3.79 percent byweight tantalum, about 18.0 percent molybdenum, about 0.09 percent byweight carbon, about 0.6 percent by weight silicon, about 0.3 percent byweight manganese, about 3.3 percent by weight iron, about 10.5 percentby weight cobalt, and the balance nickel.

References Cited UNITED STATES PATENTS 2,777,776 1/1957 Binder -1713,164,465 1/ 1965 Thielemann 75-171 3,203,792 8/ 196 5 Scheil et a175-171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75-134

